The present invention is directed to integrated circuits and, more particularly, to a band gap reference voltage generator.
Reference voltage generators are used widely in integrated circuits (IC) and other electronic circuits to provide a reference voltage that is stable despite variations in fabrication processing conditions from one batch of products to another, and despite variations in operating temperatures. Various techniques are available for compensating the reference voltage for process variations, such as including trim resistors in the circuit design, which can be set or ‘trimmed’ when producing the IC.
Thermal compensation is commonly obtained by including a band gap module in the reference voltage generator. A band gap module includes forward-biased semiconductor PN junctions, which may be provided by diodes or by diode-connected bipolar junction transistors (BJT) or metal-oxide semiconductor field-effect transistors (MOSFET), for example. The voltage across a forward-biased semiconductor PN junction for a given current through the junction decreases with increasing temperature, commonly called complementary to absolute temperature (CTAT), varying by approximately −2 mV/° K in a silicon semiconductor, for example. A band gap module uses a voltage difference between a pair of matched forward-biased PN junctions operating at different current densities to generate a current that increases with increasing temperature, commonly called proportional to absolute temperature (PTAT). This current is used to generate a PTAT voltage in a resistor that is added to a CTAT voltage across a semiconductor PN junction, which may be one of the matched pair. The ratio of the PTAT and CTAT voltages may be set by setting resistance values, for example, so that the temperature dependencies of the PTAT and CTAT voltages compensate each other to a first order approximation. Typically, in a semiconductor device, the resulting voltage is about 1.2-1.3 V, close to the theoretical band gap of silicon at 0° K, 1.22 eV. The residual second order approximation of the temperature dependency typically is small within the operating temperature range around the temperature at which the ratio of the PTAT and CTAT voltages is set.
Trimming resistance values for the band gap module is conveniently performed digitally by setting switches or fuses to connect or short circuit trim resistors. It is desirable to be able to trim the resistance values bidirectionally about a central value, which is not the case in some known implementations. In some conventional implementations, it is necessary for the ON resistance of the trim switches to be small to reduce inaccuracy introduced by variability of their ON resistance, for example with variation of supply voltage. Trim switches with small ON resistance in conventional implementations tend to occupy a large area of the IC.